A color graphics printer generally includes a printer frame buffer in which information representing the desired distribution of color over a printable area of a record medium, such as a sheet of paper, is stored in CMY format. It is known to use a binary printer, e.g. an ink jet printer or a thermal wax transfer printer, for color graphics printing. A binary printer applies discrete dots of color media to the record mediun and addresses the record medium in accordance with a square array of discrete record location, rather than over a continuum of locations. The dots are all of the same size, and variation in the darkness with which a color medium is applied to the record medium is accomplished by varying the number of dots applied over a unit area of the record medium. The addressable memory locations in the printer frame buffer are associated on a one-to-one basis with the addressable record locations of the record medium, and each addressable memory location is capable of storing three digital words each comprising, for example, eight binary digits, or bits. Each set of three digital words represents the contributions of the three primary color components (cyan, magenta and yellow) to the desired color of the associated record location of the record medium. (An additional word may be associated with the color black.)
When a printable record medium is addressed, the printer scans successive rows of record locations in the square array. The contents of the printer frame buffer are read synchronously with scanning of the record medium by the printer. The record locations of the record medium are organized into contiguous dither cells, each of which might have sixteen record locations on a side, and thus contain 256 record locations. A unique gray scale reference value, which may be expressed as an integer in the range from 0 to 255, is associated with each record location within the dither cell. The three (or four) digital words associated with a particular record location are compared with the reference value for that location, and if one of the digital words represents a value that is at least as great as the reference value, the printer applies a dot of the appropriate color to the record medium. All the dots are nominally of the same size, but in practice the dots are usually larger than their nominal size. For example, in the case of an ink jet printer, the ink tends to bleed over the record medium, and in the case of a thermal wax transfer printer the dots tend to smear.
Record locations having successive reference values are not grouped together in a dither cell but are distributed over the cell in accordance with a predetermined dither pattern. The dither pattern may be based on a magic square. A magic square is a square array of P.times.P terms in an arithmetic progression such that the sum of the terms in each column, each row and each major diagonal is the same. In a magic square, containing P.times.P integers from O to P.times.P-1, successive numbers are distributed in a substantially random fashion over the square. A magic square can be defined for P=4.
In a color graphics printer that is at present commercially available, the 256 record locations in a 16.times.16 dither cell are organized as a 4.times.4 magic square of groups, each of which contains a 4.times.4 magic square of record locations. In this case, the sixteen groups contain the numbers from 0 to 15 respectively as their smallest reference values, and the sixteen groups are arranged in a magic square in accordance with their smallest reference values, as shown in Table I. The fifteen additional numbers in each group are in an arithmetic progression having a common difference of 16, and the sixteen numbers in each group are arranged in a magic square, as shown in Table II for the group containing the number 0.
TABLE I ______________________________________ 1 12 2 15 6 11 5 8 13 0 14 3 10 7 9 4 ______________________________________
TABLE II ______________________________________ 16 192 32 240 96 176 80 128 208 0 224 48 160 112 144 64 ______________________________________
With the type of dither pattern described with reference to Tables I and II, successive reference values in a group of record locations are distributed over the group in a substantially random fashion. If a printable record medium is printed using a dither pattern of this type, for gray scale levels below about 128 (in the range from 0 to 256), the enlarging of each dot due to bleeding or smearing has the effect of increasing significantly the darkness of the image created on the record medium beyond the value indicated by the contents of the frame buffer.